1. Field of the Invention
This invention relates to a projection television apparatus using a green cathode ray tube having an improved color reproduction characteristic.
2. Description of the Related Art
FIG. 7 of the accompanying drawings shows a typical conventional projection television apparatus in which a cathode ray tube equipped with a multilayered interference filter is used. As shown in FIG. 7, the projection television apparatus comprises a cathode ray tube 1, and a series of projecting lenses 2 disposed in front of the cathode ray tube 1. Element 3 designates an incident pupil position of the series of projection lenses 2; 4, a display face plate pane; 5, a multilayered interference filter; 6, a fluorescent material; 7, a vacuum evaporation aluminum film.
The operation of this conventional projection television apparatus will now be described. FIG. 4 shows a characteristic curve 9 of a luminous spectrum of a multilayered interference coating with respect to transmittivity as an electron beam is incident on the fluorescent material 6 coated on the cathode ray tube 1. In view of its life and brightness, any existing fluorescent material to be used in a projection television apparatus has inevitably such luminous spectrum. As shown in FIG. 4, the spectrum includes, in addition to an essential green spectral wavelength of 545 nm, blue-side and red-side spectra. Therefore, as shown in a chromaticity diagram of FIG. 5, only the chromaticity value at a point G.sub.0 16 remote from the standard green chromaticity point G13 can be obtained; therefore, color reproduction is possible only within a triangular range 19 indicated by broken lines.
To this end, it is a known practice to place, between the face plate pane 4 and the fluorescent material 6, a multilayered interference filter 5 composed of a plurality of alternately superimposed layers of high and low refractive index materials, and then to determine the transmittivity characteristic of this multilayered interference filter as indicated by the broken lines 11, so that unnecessary light spec
having peaks around 600 nm of the light spectrum 9 emitted from the green fluorescent material 6 are reflected. Therefore it is possible to improve the color reproduction up to the chromaticity point G.sub.1 17 of FIG. 5. However, as the transmittivity characteristic of the interference filter 5 is determined as indicated by the broken lines 11, it is impossible to reflect the unnecessary light spectrum having a peak at a wavelength of slightly less than 500 nm. As a result, color reproduction can be possible yet in the triangular range 20 indicated by the dash-and-one-dot lines in FIG. 5.
Since the image on the cathode ray tube 1 is disposed at an angle of .THETA..sub.1 with respect to the incident pupil 3 which is at the center of the projection lens series 2, the difference l in optical light path between the layers of the multilayered interference filter 5 can be expressed by the following equation: EQU l=2d cos .THETA..sub.1 (d: layer thickness).
Consequently, the transmittivity characteristic, in the direction of angle .THETA., of the multilayered interference filter 5 will, as indicated by dash-and-one-dot lines 12 in FIG. 4, cause even the necessary green light spectrum to be reflected, thus impairing the color reproduction range.
With this conventional arrangement, partly because the range of color reproduction is inadequate for light from the central areas of the cathode ray tube, and partly because light from the peripheral areas is reflected up to the essential light spectrum, the display color would become blue-emphasized green.